Squid is a caching proxy for the Web supporting HTTP, HTTPS, FTP, and more. It reduces bandwidth and improves response times by caching and reusing frequently-requested web pages. Squid has extensive access controls and makes a great server accelerator. It is licensed under the GNU GPL.

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If you are looking to setup a transparent squid proxy, see this page

  • 1Terminology
    • 1.2proxy
  • 3Basic configuration
    • 3.2Testing
  • 4SSL interception or SSL bumping
    • 4.3Configuration
      • 4.3.2Generate cert/key pair



A client is often considered a user of a PC or similar system, but more accurately a client is the applications a person uses to access web pages and other resources, and the OS they are running on.


A proxy is a device which makes connections on behalf of clients. If we consider a common TCP connection, there is one TCP connection between the client (source) and the proxy, and a separate TCP connection between the proxy and the server (destination). Consider this beautiful diagram:

Point A is the client-side connection and point B is the server-side connection.

These are separate, distinct connections, so for example the client-side could be encrypted and the server-side in plaintext (unencrypted), or the client-side could use browser user-agent header x and the server-side connection could use browser user-agent header y.

The proxy is effectively acting as a server to the client, and as a client to the server (OCS). Without a proxy, the connection would simply be from client to server. The destination server is often referred to as the 'OCS' or 'Origin Content Server' - this simply means the server hosting the objects that the client requests (for example the web pages that you want).

The above is of course a simplified version of things. Other factors, such as the HTTP version of the client browser, or existence of the object in cache on the proxy, will have impacts on how many server-side connections are created.

explicit forward proxy

An explicit proxy is one in which the client is explicitly configured to use the proxy, and as such are aware of the existence of the proxy on the network. When the client sends packets to an explicit proxy, they are addressed to the proxy server listening address and port.Squid usually listens for explicit traffic on TCP port 3128 but TCP port 8080 is a common explicit proxy listening port. Explicit proxy deployments are forward proxy deployments, where the clients can make use of the caching and optimisation features of the proxy when making outbound requests. An explicit proxy can be involved in authentication of the client, typically by issuing a 407 HTTP response. Connections to HTTPS sites through an explicit proxy will use the CONNECT HTTP method.This article discusses this type of proxy deployment.

transparent forward proxy

A transparent proxy, also known as an intercepting proxy, does not require any configuration changes on the client, since traffic is transparently sent to the proxy, usually through traffic redirection by a router. When the client sends packets, they are addressed to the destination server. A transparent server can be involved in client authentication; this will usually involve redirecting the client to a virtual domain, typically with a 401 HTTP response.

reverse proxy

A reverse proxy sits in front of a resource such as a web server and answers queries from clients, caching content from the server and optimising connections to it.


A cache is simply an object store. A proxy will usually cache objects (images, html text, downloaded files etc) that are requested by clients, which means storing the objects on the proxy either in RAM or on disk.This has the benefit of a client being able to get the object from the proxy, without having to wait for the proxy to connect out to the destination server (OCS) and download the object again, resulting in a better client experience ('the web pages seem to load faster') and reduced bandwidth (less connections made server side, especially if we consider objects that are repeatedly requested).

Caching is influenced by proxy configuration (what to cache) and by numerous HTTP headers (am I allowed to cache this object? How long should I cache it for?) such as 'Expires', 'Cache Control', 'If-Modified-Since' and 'Last-Modified'.A proxy will usually keep its cache fresh by making requests for cached objects independent of client requests for the objects.

More information

You may also wish to review https://devcentral.f5.com/articles/the-concise-guide-to-proxies which provides further information on various proxy types.


Install the squid package:

apk add squid

If you wish to use the Alpine Configuration Framework (ACF) front-end for squid, install the acf-squid package:

apk add acf-squid

You can then logon to the device over https://x.x.x.x (replace x.x.x.x with the IP of your server of course) and manage the squid configuration files and stop/start/restart the daemon etc.

Basic configuration

Config file

The main configuration file is /etc/squid/squid.conf. Lines beginning with a '#' are comments.squid should already come with a basic working configuration file but an example configuration file is shown below, which will get you up and running quickly and is well commented but please change the localnet definition for a more restrictive one:

Note:If you change the squid configuration file, you do not need to restart squid in order to load the changes, just use this command instead:

squid -k reconfigure


Start and check squid

Start the squid service:

rc-service squid start

To start squid automatically at boot:

rc-update add squid

Check the squid configuration for errors:

squid -k check

If there is no feedback, everything is gravy! (that's a good thing).

Check that squid is listening for traffic, using netstat for example:

netstat -tl

You should see a line showing a Local Address and the listening port (in our example config above it is set to 3128). If you don't see this, check the 'http_port' directive is set in the config file and has a value. Ensure this port isn't being used by something else on the system.

Remember to ensure the squid proxy has valid IP configuration including default gateway etc.

Configure the client

Each application using the proxy will have to be configured to send traffic via the proxy. If we assume that our squid proxy is running on IP address, port 3128, we would configure the Firefox browser in the following manner:

  • Tools>Options>Advanced>Network>Settings..
  • Select Manual proxy configuration and tick the 'use this proxy server for all protocols' box
  • Under HTTP Proxy: add the squid listening IP address, In the Port: section add the squid listening port 3128
  • Click OK to save the changes.

Now browse, you should have internet access, via the proxy!

Many Operating Systems allow a system proxy to be set. Firefox can be set to use the system proxy settings:

  • Tools>Options>Advanced>Network>Settings..
  • Select Use system proxy settings
  • Click OK to save the changes.

The system proxy settings themselves vary from system to system but on an Alpine install you can simply run the setup-proxy script.

It is also possible to configure the browser to use a PAC file. This file is usually hosted on a webserver (which may also be the proxy, but doesn't have to be) and it tells the browser what requests to send to the proxy and which ones to send direct (bypassing the proxy).

The Squid FAQ on configuring browsers offers more information on this topic.


If you've set the proxy to take access logs, you can view these to see client requests coming in:

tail -f /var/log/squid/access.log

Use Ctrl-C to exit back to the prompt.

SSL interception or SSL bumping

The offical squid documentation appears to prefer the term SSL interception for transparent squid deployments and SSL bumping for explicit proxy deployments. Nonetheless, both environments use the ssl_bump configuration directive (and some others) in /etc/squid/squid.conf for their configuration.In general terminology, SSL interception is generally used to describe both deployments and that will be the term used here. We are, of course, dealing with an explicit forward proxy configuration here.

Behaviour without SSL interception

Clients behind an explicit proxy use the 'CONNECT' HTTP method. The first connection to the proxy port uses HTTP and specifies the destination server (often termed the Origin Content Server, or OCS). After this the proxy simply acts as a tunnel, and blindly proxies the connection without inspecting the traffic.

Behaviour with SSL interception

Using this method, clients still use the CONNECT method. Typically the server-side connection is established first, using the information available in the CONNECT request from the client (such as the destination server and port) which allows Squid to spoof a certificate. The Common Name (CN) will reflect the destination server and the Squid certificate will be used to sign it. This spoofed certificate is then presented to the client when they access a site via the proxy.


Add packages

Add the ca-certificates package (required to trust common Certificate Authority (CA) certificates) and the openssl package or libressl package (to create self-signed certificate or CSR). The -U option ensures we update the package list first:

apk -U add ca-certificates libressl

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Generate cert/key pair

You obviously don't need to follow both of the next sections. Either generate a self-signed certificate or a CA signed one (you have to pay for the latter) and then amend the squid configuration to enable SSL interception and point it to the key/cert pair generated in these steps.Whether you are using OpenSSL or LibreSSL, the command to use is still 'openssl'.

Generate a self-signed certificate

The following example command will produce a working cert/key pair, saved to /etc/squid/squid.pem:

openssl req -newkey rsa:4096 -x509 -keyout /etc/squid/squid.pem -out /etc/squid/squid.pem -days 365 -nodes

Then adjust permissions:

chmod 400 /etc/squid/squid.pem

In the above example we save the cetificate and key to the same file; they can be saved to separate files if you wish, just adjust paths accordingly.

Generate a CSR to get a CA-signed certificate

Create a private key using the syntax openssl genrsa -out <key_path_and_name> <keysize>

For example:

openssl genrsa -out /etc/squid/squid.key 2048

Create the CSR with the syntax openssl req -new -key <key_path_and_name> -out <csr_path_and_name>

For example:

openssl req -new -key /etc/squid/squid.key -out /etc/squid/squid.csr

You then need to supply the CSR (Certificate Signing Request) to your Certificate Authority (CA). Do not send them, or anyone else, your private key. It should remain private!

Some CA's (such as Thawte and Verisign) provide an online CSR checker, so you can ensure the CSR is valid before providing it to them.

Once the CA receive the CSR and do their thing they should send you back the CA signed public key. Request it in .pem format if possible (it's a widely used standard for certs). You then need to copy this back onto the Squid proxy, to /etc/squid/ if you are following the example here.

Remember to amend the squid configuration to point at the correct locations of the private key and CA signed certificate.

Amend /etc/squid/squid.conf

Next, we need to amend the squid configuration file to use SSL interception. In the below example, we will add a few lines, then amend the http_port directive so that it still serves HTTP requests, but also performs SSL interception on HTTPS connections that are established via the HTTP method CONNECT. You can use a separate http_port for each if you wish, but remember to amend the client configuration to send HTTPS traffic to the alternative port.

The decision you're making with the 'sslproxy_cert_error' (and potentially the 'sslproxy_flags') option is to either close the connection when a certificate error is encountered (such as a self-signed, untrusted certificate is presented), or to pass certificate errors onto the client to allow them to make the choice about the site and whether or not to trust the certificate.

Fix client SSL Warnings

3utools linux. You will need to install the self-signed proxy certificate (in our example we saved it to /etc/squid/squid.pem) to all clients, otherwise they will probably get an SSL error for every domain they visit over HTTPS. It is important to install the certificate as a Certificate Authority (CA) certificate to the client browser for trust to establish properly. If you are using a CA signed certificate (not a self-signed one) then the browser probably already trusts the certificate and so this step will likely not be needed.

For Internet Explorer, you can likely double-click the .pem certificate and use the Certificate Import Wizard to manually install the certificate to the Trusted Root Certification Authorities certificate store.

For Firefox, use Tools>Options>Advanced>Certificates>View Certificates. Under the Authorities tab, use Import.. to add the certificate as a trusted authority. Installing the certificate as any other kind of certificate will result in a poor user experience.

Note:If you see the error This certificate is already installed as a certificate authority. be sure to check all locations for existence of the certificate and remove (delete) it wherever found. Firefox likes to install certificates as Server certificates rather than under Authorities as we would like. Once removing all traces of the certificate, please be sure to restart Firefox and try importing the certificate again.

Disable SSL interception for certain sites

There may be situations where you wish to disable SSL interception/SSL bumping for certain destinations due to issues with functionality or privacy concerns. As an example, a Windows Dropbox client refused to establish a secure connection because it did not trust the self-signed certificate in use by the proxy. Or, you may wish to allow user privacy to be retained when they are using hotmail.com.In this example we will create an Access Control list (ACL) to prevent SSL interception to *.hotmail.com and *.dropbox.com. Remember that rule order is important, the first match wins! So put more specific rules at the top, more general rules below.

Squid crashes after configuring HTTPS interception

Squid may crash after configuring SSL interception. The service may report as running, but reviewing listening ports no longer shows Squid listening.A review of /var/log/messages may show an error 'The ssl_crtd helpers are crashing too rapidly, need help!'

In this instance, perform the following:

Further reading

Advert blocking

There are several methods to achieve this, you could simply create an ACL for known advert domains (see blocking domains for an indication of how to do this).Another options is to use a hosts file specific to squid (i.e. unrelated to the system hosts file), which will direct traffic for known adverts/malware sites to the localhost. The connection can then either fail (because there is no web server running at to service the requests that are redirected by the hosts file to localhost) and squid will display a standard error, or you can have a web server running that will respond with some form of 'advert blocked' page.

Blocking ads will save bandwidth and should improve page load times. As a drawback, some pages may look untidy or odd without advertising in place.

The first thing to do is either create a hosts file yourself or find a pre-configured one such as this one (note that this file is free to use for personal use only, see the full license here.

Whichever method you choose, save the hosts file to the local filesystem, in our example to /etc/squid/hosts.txt

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Then, add the hosts_file directive to the squid configuration:

Remember to reload the configuration/restart the squid service for the changes to take effect.

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Blocking domains

If you have a large number of domains you wish to block, instead of adding them directly to the squid configuration file the best option is to create a separate list and reference this in the configuration file.The domain list should have domains listed one per line. There is an example list (warning, this doesn't get updated!) available here We will refer to this list in our example below.

- Create your own, or download a domain list and save it to /etc/squid/porndomains.acl:

- Amend the squid configuration file at /etc/squid/squid.conf as follows:

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- Check the squid configuration for errors:

squid -k check

and if there are none, apply the changes:

squid -k reconfigure

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- Done! Domains from the list should now be blocked.

You can of course create your own lists of domains and add blacklists/whitelists to your configuration based on the above example. Each list should of course have a unique name.

DNS configuration

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No additional DNS configuration is required since by default Squid will use the settings in /etc/resolv.conf.You may wish to change this behaviour for your environment or tweak settings to improve performance, as per the below example. It's heavily commented as always for my examples, change to suit your needs.

More information

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